well; after trying a bunch of props; here's a rundown on the results....
boat needed 4200-4600 RPM at WOT

Original prop was a SS 14x19 3 blade -- WOT 3800 rpm

aluminum 15x17 3 blade - WOT 4950
aluminum 14.5 x 19 3 blade WOT 4250

the 4 blade dropped it 150; so it was too low for for reasonable use; but
getting on plain & staying there was great.

The difference in RPM from the SS to Alum 15x17 is being attributed to the more
agressive cupping on a SS blade.

so I'm much happier with the alum 14.5x19 -- but will try some some 4 blade
15x18 props to see how they do ... but that won't be until much later in summer.

Thought it was interesting about the difference in cupping would make that much
of a difference.